The sun will set Tuesday on Ulysses, a robotic sentry that spent more than 18 years exploring unseen reaches of the sun for NASA and the European Space Agency.

Credit: ESA

Ground controllers will communicate with the nuclear-powered probe for the final time Tuesday, ending a $1.4 billion mission that operated more than three times longer than expected.

"It's time to call it quits and wave a fond farewell," said Nigel Angold, ESA mission operations manager based at the Jet Propulsion Laboratory in Pasadena, Calif.

Ulysses is running on fumes, sending back less science data, and in danger of freezing to death, according to mission officials.

"I feel ambivalent," said Ed Smith, NASA project scientist at JPL. "It's been such a long time and it's always sad. You get quite devoted to the spacecraft and to the other members of the team."

NASA launched Ulysses aboard space shuttle Discovery in October 1990. Spinning and balancing like a top, the probe was boosted into the outer solar system by two upper stage rockets after deploying from Discovery.

Ulysses flew by Jupiter in 1992, using the giant planet's immense gravity to naturally propel it out of the solar system's ecliptic plane and into a polar orbit around the sun.

The spacecraft was built by Dornier Systems of Germany, now part of the Astrium consortium.

ESA oversees operations of Ulysses from JPL, while NASA is in now in charge of ground systems and data processing.

Although the spacecraft does not have a fuel gauge, engineers predicted the solar probe should have run out of hydrazine propellant months ago. Ulysses uses hydrazine to fire thrusters keeping its high-gain antenna pointed toward Earth.

The craft's nuclear power system, called a radioisotope thermoelectric generator, is also producing less power than it did at launch. The dwindling power supply means Ulysses must turn off some of its heaters to operate critical control systems and science instruments.

One cold spot is in the plumbing leading to Ulysses' maneuvering jets, sparking fears that the hydrazine would freeze in its pipes, crippling the 735-pound spacecraft.

Engineers devised an ingenious method to keep the fuel from freezing. Like homeowners who keep their sink faucets dripping on cold nights, officials ordered Ulysses to fire its thrusters every two hours to keep hydrazine flowing through pipes.

This unplanned use of hydrazine depleted the spacecraft's fuel tank. It is a matter of weeks before all the fuel is surely gone, despite predictions that Ulysses should have exhausted its propellant months ago, officials said.

Estimates derived from subtracting the fuel used from the amount loaded aboard the spacecraft before launch put the remaining quantity below zero.

The mission hit major trouble in January 2008, when its X-band transmitter failed. Controllers were testing the high-speed communications system by turning it off and back on as part of an engineering exercise.

"The problem was it never came back on again," said Richard Marsden, ESA mission manager and project scientist.

Engineers believe the failure was caused by a power supply problem that was not detected until the transmitter was turned off. The lost transmitter also helped keep the hydrazine lines warm, fueling concerns about freezing propellant.

The failure forced engineers to route all communications through the less powerful S-band communications system, drastically reducing the rate data could be exchanged with Ulysses.

Controllers must use massive 70-meter, or 230-foot, dish antennas part of NASA's Deep Space Network to receive scientific data from Ulysses. Smaller 34-meter, or 112-foot, antennas are only good for uplinking commands, according to mission officials.

The tracking network has stations in Madrid, Canberra, Australia, and Goldstone, Calif.

The problem: The huge antennas are in high demand, especially by active spacecraft such as the newly-launched Lunar Reconnaissance Orbiter. On top of that, one of the antennas has been down for long-term maintenance, officials said.

During the past few months, Angold said scientists have been receiving about 5 percent of the data collected by Ulysses, down from a high of more than 95 percent during the prime of the mission.

The mission began as an "Amazon expedition" just to see what was going on at the poles, said Arik Posner, Ulysses program scientist at NASA headquarters.

Credit: ESA

Ulysses observed the sun during nearly two complete solar cycles, including a reversal of the sun's magnetic field. The sun fluctuates from low to high activity during the cycle, which lasts about 11 years.

The joint mission was also the first to directly study the sun's poles, a dream of scientists dating back 50 years.

Solar activity can affect Earth, particularly during violent solar storms that can damage electrical grids, interfere with radio signals, and incapacitate satellites critical to infrastructure.

The spacecraft's ten instruments sensed the lightning fast solar wind constantly emanating from the sun. The detectors conducted in situ measurements of ions and electrons, dust grains and gas, magnetic fields, radio and plasma waves, X-rays, and gamma rays.

"We have learned an enormous amount about the solar wind, where and how it originates," Smith said. "We've learned a great deal about the heliosphere itself and how energetic particles get accelerated inside the heliosphere, and the effect the heliosphere has on cosmic rays coming in from the galaxy."

Ulysses' unique orbit and long life made the scientific rewards possible.

"This is something which you cannot really do with a spacecraft that is either short-lived or is not in an orbit where you can really measure the three-dimensional distribution of all these phenomena," Marsden said.

"Looking inward and outward, we have really covered an enormous range of science with Ulysses, not only about the sun but also about the local interstellar neighborhood," Marsden said.

NASA and ESA are reviewing plans to launch two independent missions to the sun in the next decade, continuing the legacy of joint solar observations begun with Ulysses.

If launched, the Solar Probe and Solar Orbiter missions would approach the sun closer than ever before and also study the polar regions in more detail.

Ulysses turned off due to diminishing science, threat of freezing fuel

NASA and ESA first announced the coming end of the mission in February 2008, one month after Ulysses lost its X-band transmitter.

Officials thought the hydrazine would soon freeze and the diminishing science return would compel a quick end to the wildly successful mission.

"This time last year, we literally did expect the mission to terminate within a number of weeks," Marsden said. "Even the most optimistic of us have been surprised by just how long the mission has continued."

But engineering ingenuity and the state of solar activity kept Ulysses valiantly hanging on, much like its namesake, the Greek king featured in Homer's epic poems.

"It's one of those stories where the spacecraft is really living up to its name," Marsden said.

The fuel bleeding procedure kept the hydrazine from freezing as expected, and scientists desperately wanted measurements of the sun during an unusually quiet period known as solar minimum.

"The sun is as quiet as it has been for the whole Space Age," Angold said. "The science is quite compelling."

Marsden said the current solar minimum could be quietest in the past 100 years, maybe even longer.

The rich scientific outlook was an "incentive" to continue the mission past its scheduled end last July, Angold said.

Credit: ESA

The date for the end of data collection was reset for the end of March, but the project had enough money to continue the mission through September, according to NASA.

NASA funding for operations this fiscal year was $1.5 million, according to an agency spokesperson. ESA provides about $3.5 million per year, according to European officials.

But decreasing DSN tracking time, the threat of frozen fuel, and exhausted funding reserves meant project managers could no longer make the "technical case" to push on.

"I've been one of those people who tried to keep things going and keep things going, but I think now is the time," Angold said. "It's very much a case of diminishing returns."

Ulysses is nearing the sun's equatorial region in its six-year-long orbit, an area well-covered by other NASA spacecraft, according to Posner.

The probe would not be in a favorable position for polar observations for several years, well past the most optimistic guesses of when the fuel would run out.

"There were various issues that had to do with the declining data rate ... the (poor) position of Ulysses in terms of latitude and the outlook of not being able to get it back after that," Posner said.

"We don't have much fuel, and this gives us a way to switch it off in a graceful fashion, rather than it just happening," Angold said.

Ulysses lived much longer than anyone could have ever predicted, cheating death for a year after its ground team nearly gave up hope.

"I really don't see it as anyone switching the spacecraft off early," Angold said. "We're more than 12 years past the mission's design lifetime. It's just time now."

Ulysses' final hours

Tuesday's five-hour communications pass with Ulysses will be a poignant moment for dozens of scientists gathered inside the Ulysses control room in California.

The ground team invited former members to be present for the final moments of a project that was started more than 30 years ago.

"I think it will be very emotional," Posner said. "There will be all these pioneers who worked on the instruments and who got the spacecraft through the two agencies."

Many of the workers still on Ulysses have careers intimately linked to the mission.

"Most people have had their offices within 10 or 20 feet of each other for the past 19 years," Angold said.

Ed Massey, NASA's Ulysses project manager, started working on Ulysses in 1987, three years before launch.

"I've had a chance to see this mission from prior to launch until the end," Massey said. "It's been quite a fulfillment."

Smith became NASA project scientist in 1977 and was the first U.S. member of the joint team.

"I've kidded to people at JPL that I wanted to be the last person here to turn off the lights," Smith said.

Marsden traces his involvement with Ulysses back 33 years. He became project scientist in 1992.

Angold joined the mission in 1989, just before Ulysses launched.

"There will be four or five of us actively involved in commanding the spacecraft (Tuesday) that were in the room from the beginning," Angold said.

Tuesday's events will begin at 1535 GMT (11:35 a.m. EDT), when the tracking dish antenna in Madrid locks on to the signal from Ulysses.

Controllers will first turn off electronics overseeing the thruster firings that control the probe's attitude, or orientation in space.

Commands will next be sent to turn on heaters near Ulysses' maneuvering jets as more power becomes available when other systems go offline.

Engineers next will order Ulysses to stop the fuel bleed procedure, the timed thruster firings designed to keep hydrazine from freezing.

The control team will also turn off the probe's tape recorder, which has recently been used as a makeshift heater to warm certain parts of the spacecraft.

Orders telling Ulysses to reconfigure its communications system will have already been sent to the spacecraft early Tuesday. The commands will be stored aboard Ulysses' computer to be executed at a specific time.

Ulysses will connect its receiver and transmitter to low-gain antennas to allow future engineers to communicate with the spacecraft, if so desired. The low-gain antennas can detect and send signals to a wide swath of the sky, meaning dishes on Earth do not have to lock on to Ulysses.

But the omni antennas are only useful much closer to Earth. Ulysses' next close approach is slated for 2013.

Future contact is unlikely and would probably be unproductive because the probe's fuel will certainly be frozen.

"The receivers will be on, but the transmitters will be off," Angold said. "It will be no longer talking to us, but it will be listening."

Controllers will confirm the transition to low-gain antennas at about 1945 GMT (3:15 p.m. EDT), 45 minutes after the command is scheduled to be executed. It takes one-and-a-half hours for radio signals to reach to and from Ulysses, which is nearly 500 million miles from Earth.

"Finally, the transmitter on board the spacecraft will be turned off," Angold said.

"And that will be the end."

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